Numerous conveyor systems having a variety of features are generally known for conveying PC boards (U.S. Pat. Nos. 4,969,552 and 4,874,081, and PCT/DE 86/00457), silicone wafers U.S. Pat. Nos. 4,217,977, 4,591,044 and 4,457,419), optical discs (U.S. Pat. No. 4,512,455), and other sheet materials (U.S. Pat. Nos. 3,948,385, 4,727,980, 4,484,675 and 4,967,898).
The turnover devices, belt tensioners and belt changing mechanisms of these and other prior art conveyor systems often require relatively complex apparatuses to perform fairly simple functions. Such designs are disadvantageous since they may unnecessarily complicate manufacture, operation and repair, as well as increase the cost, of conveyor systems. Turnover devices, belt tensioners and belt changing mechanisms having less complex designs are also known, but require relatively frequent replacement of parts and may suffer other functional deficiencies.
Known relatively complex turnover devices include those disclosed in U.S. Pat. Nos. 3,948,385, 4,512,455 and 4,969,552. The turnover device of U.S. Pat. No. 3,948,385 comprises upper and lower arms 4a,4b and swingable clamp arm 5 of a rotatable arm member assembly 4. The device of U.S. Pat. No. 4,512,455 includes a pair of coaxially mounted cam plates 52 and arm support wheels 40. Pickup arms 42 are spring biased toward conveyor 16 by caming projections 54 as support wheel 40 rotates in order to receive a disc. Pickup heads 58 are slidably mounted on arms 42 with bias from springs 70. The turnover device of U.S. Pat. No. 4,969,552 comprises a pair of channels 70 having milled slots, follower wheels 76,78 for guide 64, channel blocker 80, and a central pivot 88 connected to a slide 16. The device includes a vertical bar 12 upon which slides 16 rise permitting inversion of channels 70.
Less complex turnover devices, such as that disclosed in U.S. Pat. No. 4,484,675, simply lift an article through more than 90 degrees with curved turner arms 10 and permit it to pivot and fall back onto the conveyor. Devices such as this and other relatively simple designs disadvantageously do not enable articles to be relatively precisely located upon turning them over, and also may damage relatively fragile or sensitive articles.
Known relatively complex belt tensioner designs include those disclosed in U.S. Pat. Nos. 4,727,980 and 4,217,977. The belt tensioner of U.S. Pat. No. 4,727,980 requires lower beams 38,39, and internal pulley supports 43,44 each having first and second adjusting screws 41,42 and 45,46. The belt tensioner of U.S. Pat. No. 4,217,977 requires double conveyor pulleys 33,39, first and second sets of upper and lower idler pulleys 57,59 and 61,63, and a horizontal idler pulley 67. At least one of the pulleys in the first and second sets must be mounted on an axis skewed from the others.
Simpler belt tensioning mechanisms include "floating" drives, counterweight systems, and other variable tensioners. U.S. Pat. Nos. 1,476,936 and 4,991,984 disclose tensioners utilizing springs for tensioning belts. Spring tensioners may apply imprecise and variable tension to belts which may be disadvantageous for carefully engineered conveyor systems requiring precision belt speed. Such conveyors are useful to repeatedly deliver articles at a particular location along the conveyor. Also, springs may require periodic replacement.
U.S. Pat. No. 4,287,983 discloses a belt changing mechanism wherein endless chains are each carried in unitary cassettes. The chains are independently and individually removable and replaceable as a unit with the cassettes. Other means of changing belts may, for example, involve some degree of time-consuming disassembly or realignment of parts of the conveyor.